Heme oxygenase (HO-1) provides a cellular defense mechanism during oxidative stress and catalyzes the rate-limiting step in heme metabolism that produces biliverdin (BV). The role of BV and its potential use in preventing ischemia/reperfusion injury (IRI) had never been studied. This study was designed to explore putative cytoprotective functions of BV during hepatic IRI in rat liver models of ex vivo perfusion and orthotopic liver transplantation (OLT) after prolonged periods of cold ischemia. In an ex vivo hepatic IRI model, adjunctive BV improved portal venous blood flow, increased bile production, and decreased hepatocellular damage. These findings were correlated with amelioration of histological features of IRI, as assessed by Suzuki's criteria. Following cold ischemia and syngeneic OLT, BV therapy extended animal survival from 50% in untreated controls to 90% to 100%. This effect correlated with improved liver function and preserved hepatic architecture. Additionally, BV adjuvant after OLT decreased endothelial expression of cellular adhesion molecules (P-selectin and intracellular adhesion molecule 1), and decreased the extent of infiltration by neutrophils and inflammatory macrophages. BV also inhibited expression of inducible nitric oxide synthase and proinflammatory cytokines (interleukin 1, tumor necrosis factor ␣, and interleukin 6) in OLTs. Finally, BV therapy promoted an increased expression of antiapoptotic molecules independently of HO-1 expression, consistent with BV being an important mediator through which HO-1 prevents cell death. In conclusion, this study documents and dissects potent cytoprotective effects of BV in well-established rat models of hepatic IRI. Our results provide the rationale for a novel therapeutic approach using BV to maximize the function and thus the availability of donor organs. (HEPATOLOGY 2004;40:1333-1341 I schemia and reperfusion injury (IRI), an antigen-independent component of the insult to the liver during "harvesting," represents an important problem affecting liver transplantation. IRI causes up to 10% of early liver failures and can lead to a higher incidence of acute and chronic rejection. 1 Moreover, with the increasing donor shortage, more functionally "suboptimal" or "marginal" livers are being used. Such livers are more susceptible to the damage caused by IRI compared with normal livers. 2 Indeed, minimizing the adverse effects of IRI could significantly increase the number of patients that successfully undergo liver transplantation.Liver IRI is mediated by several processes that lead to hepatocellular damage, which is triggered when the liver is transiently deprived of oxygen during the organ procurement for transplantation, and later reoxygenated during reperfusion. The structural changes promoted by cold ischemia and reperfusion become more prominent with increased storage time. 3 The sinusoidal endothelial cells are very sensitive to IRI, thus affecting the delicate balance that maintains homeostasis in the microcirculation with